Evaporative cooling has been known for centuries and is well understood. And evaporative or “swamp” coolers for the home and business have been available for many decades. Mechanical refrigeration with its many advantages over evaporative coolers in moderate to humid climates has been understood and improved upon since the beginning of the last century.
Recently the particular advantages of combining evaporative cooling with mechanical cooling, and more specifically with air-cooled condensers, have been disclosed. Air conditioners that reject heat through air-cooled condensers are inherently less energy efficient than their larger counterparts that reject heat to a body of water or to condenser water and cooling towers. This advantage of water cooled over air cooled has to do with the thermodynamics or energy transfer of water evaporation compared to convective heat or energy transfer from the air-cooled condenser.
The unfortunate reality of the air-cooled condensing units found on most commercial buildings and almost all residences is that the hotter the outdoor temperature gets the less cooling capacity the unit can provide and the more electricity it will consume. The cooling capacity decrease is approximately linear with rising outdoor temperature. And since energy consumption increase is also roughly linear, the efficiency—cooling capacity divided by energy input—is hit doubly hard.
Evaporating a stream or mist of water into the airflow through the condenser is an effective means to reduce the temperature of that airflow and greatly improve the heat transfer from the refrigerant through the condenser to the cooler airflow. The evaporation increases the relative humidity of the airflow through the condenser coils, but it is of no consequence since it remains outside. There is a net increase in the consumption of water compared to the water that is typically used in the generation of electricity. However, the environmental benefits and fuel savings weigh heavily in favor of widespread adoption of the technology.
Numerous inventors have observed these advantages and have disclosed their ideas in the prior art. Most would improve the operating efficiency of the condensing unit initially at least, but they suffer various shortcomings.
Gingold et al (U.S. Pat. No. 4,028,906) discloses a misting or fogging invention that runs whenever the compressor in the condensing unit runs. It uses a fogging nozzle that sprays the water mist directly into the condenser airflow. Gingold recognizes the need to protect the condenser coil from the scaling or build-up of calcium carbonate and other minerals and shows an inline detergent dispenser. It is not likely that this would prove to be an effective inhibitor. In the best circumstance it would add operating cost and detergent would either accumulate or run-off. And water spray or mist at all operating conditions is not warranted since the maximum benefit is achieved during the hottest conditions.
Welker et al (U.S. Pat. No. 4,685,308) improves upon Gingold's invention by adding temperature sensitive response and thereby limiting misting operation to periods of time when it will be most effective. While this reduces the overall water quantity used and also the scaling, he still includes an inline filter to reduce the scale build-up. Welker also raises the possibility of using a reverse osmosis filter to remove minerals. This is quite effective, but costly for the flowrates required and labor intensive to install. It would only be economically attractive where there are other required uses for the filtered water or under critical circumstances.
Manno (U.S. Pat. No. 4,212,172) discloses a different approach that eliminates the need for inline water filters. Instead of misting or spraying the water directly onto the coils, he discloses a box with water vanes and filters. This has proved to be effective at stopping the water mist from reaching the condenser coils. Manno's filter unit, however, is a relatively complex open box attached by screws to the condensing unit. The unit utilizes an internal arrangement wherein water is directed over water vanes to create a waterfall through which turbulent air passes before reaching the filter. The system is more complicated than need be, and therefore more expensive than necessary. And its configuration requires multiple units for two, three, and four sided condenser coil units. It can also be expected to increase the airflow resistance or pressure drop across the coil. This will reduce the total airflow and increase the condenser fan power consumption. This will decrease the benefits of the water evaporation.